1. Field of the Invention
The present invention relates to a zoom lens, which is suitable for an imaging lens for an image pickup apparatus, such as a video camera, a digital still camera, a broadcasting camera, or a silver-halide photography camera.
2. Description of the Related Art
It is important that an imaging lens, used in an image pickup apparatus using a solid-state image pickup element, be a compact, high-performance, high-powered zoom lens. A rear-focusing type zoom lens that performs focusing as a result of moving units other than an object-side first unit is one type of zoom lens that satisfies this requirement.
As the rear-focusing type zoom lens, a zoom lens including, from an object side to an image side, four units, that is, first to fourth units in that order is known (refer to U.S. Pat. Nos. 5,963,378 and 6,166,864). The first unit has a positive refractive power. The second unit has a negative refractive power. The third unit has a positive refractive power. The fourth unit has a positive refractive power. In the zoom lens including four units, the second unit is moved to change magnification, and the fourth unit is moved to perform focusing and displacement of an image field, resulting from a magnification change.
A zoom lens including, from an object side to an image side, five units, that is, first to fifth units in that order, is known (refer to Japanese Patent Laid-Open No. 2000-180724). The first unit has a positive refractive power. The second unit has a negative refractive power. The third unit has a positive refractive power. The fourth unit has a positive refractive power. The fifth unit has a positive refractive power.
In the zoom lens including five units, the second unit is moved to change magnification, and the fifth unit is moved to perform focusing.
Another type of zoom lens is proposed (refer to US Patent No. 2004/0233302 and Japanese Patent Laid-Open No. 2004-102089). In the zoom lens, a reflecting member is disposed in an optical system of the zoom lens, and a light axis is deflected by substantially 90 degrees to restrict an optical thickness in an object-side direction (that is, a front-back direction of a camera), thereby achieving a thin camera.
US Patent No. 2004/0233302 discloses a thin zoom lens having high magnification, in which a light path is bent by a prism disposed in a first unit having a positive refractive power.
In the zoom lens discussed in US Patent No. 2004/0233302, during zooming, the first unit is stationary, and a second unit and subsequent units are moved by large amounts to achieve a high zoom ratio. Therefore, the distance from a prism to an image field is increased. Accordingly, although this structure is advantageous for reducing the thickness of an optical system, the entire camera tends to become large.
Japanese Patent Laid-Open No. 2004-102089 discloses a zoom lens which is compact and has a high magnification as a result of disposing a prism in a second unit having a negative refractive power, and bending a light path by the prism.
In the zoom lens discussed in Japanese Patent Laid-Open No. 2004-102089, considering, for example, that an effective diameter of a front lens is relatively large and that conditions placing limitations on the manufacturing of a lens exist, the entire optical system tends to be large.